[#1924] Add Android voting delegation proof and submission

[greg0x]

Part of zodl-inc/zodl-android#2193.
Slice of #1924 (umbrella draft PR being split into reviewable pieces).
Stacked on #1942 (PR 4 — delegation setup + governance PCZT prep).
Resolves MOB-1109

Draft: still iterating locally; opened early so reviewers can start tracking the delegation finish.

Why

Fifth slice of the Android shielded-voting integration. PR 4 (#1942) lands the
delegation setup primitives, hotkey, and governance PCZT/sighash prep. This PR
finishes the delegation half of the workflow: enabling client-pir, persisting
witnesses, running PIR precompute, generating the delegation proof with progress
reporting, and producing the signed submission payload (including the
Keystone-supplied-signature variant).

No public Synchronizer voting API is added.

What's in

• client-pir feature on zcash_voting is now enabled; lockfile delta is mostly
  the PIR transport stack (pir-client, pir-types, valar-ypir,
  valar-spiral-rs, hyper-rustls).
• New JNI surface for the delegation finish: witness storage, PIR precompute,
  proof + progress callback, signed submission, Keystone-signature submission.
• New voting/progress.rs module hosting JniProgressReporter; the only
  per-thread JNI attach pattern in the voting backend so far.

On the VotingDbBackend seam

The typesafe Kotlin layer gains an internal VotingDbBackend interface and a
pass-through RustVotingDbBackend wrapper so TypesafeVotingBackendImpl is
unit-testable without JNI. It lands here because PR 5 is the first slice with
byte-array-typed result models that need size/count validation; PR 6–8 will
extend the same surface as more typed FFI arrives, so introducing it now
avoids a retroactive horizontal refactor later. Happy to split into a tiny
infra PR if a reviewer prefers.

Not included

• Snapshot wallet notes / note witnesses (PR 6).
• Vote tree / VAN / vote commitment / cast-vote payloads (PR 7).
• Delegation tx hash store/lookup and recovery state (PR 8).

Review focus

Primary review track: core-dev. Secondary: mobile-app-dev.

• zcash_voting client-pir feature transitive graph (Cargo.lock delta).
• PIR client construction (connect_pir_client) and SDK boundary for network
  errors.
• progress.rs threading and lifetime — attach_current_thread per callback,
  GlobalRef ownership, exception-clear after failed Java call.
• verify_delegation_submission_sig semantics for the Keystone variant: rk
  parsed as a redpallas VerificationKey<SpendAuth>, identity rejected, sig
  re-verified over the supplied sighash.
• require_witnesses_match_bundle invariants (count, root, position, duplicate
  commitments) before persisting to the voting DB.
• Re-entry guard on VotingDb: progress callbacks must not call back into the
  same handle while the access mutex is held.
• Typed DelegationProofResult / DelegationSubmissionResult byte/list size
  invariants enforced by the typesafe layer.

Note: delegationProofResultFixtureNative and storeDelegationProofFixtureNative
are JNI fixtures used by androidTest. Open to moving these behind a debug-only
artifact if reviewers prefer.

Validation

• cargo check --manifest-path backend-lib/Cargo.toml --locked
• cargo fmt --manifest-path backend-lib/Cargo.toml --check
• ./gradlew :backend-lib:compileReleaseKotlin :sdk-lib:compileReleaseKotlin
• ./gradlew ktlint detektAll
• ./gradlew checkProperties
• git diff --check

Author

• Self-review your own code in GitHub web interface
• Add automated tests as appropriate
• Update the manual tests as appropriate
• Check the code coverage report for the automated tests
• Update documentation as appropriate
• Run the demo app and try the changes
• Pull in the latest changes from the main branch and squash your commits before assigning a reviewer

Reviewer

• Check the code with the Code Review Guidelines checklist
• Perform an ad hoc review
• Review the automated tests
• Review the manual tests
• Review the documentation as appropriate
• Run the demo app and try the changes

[nullcopy]

utACK 23144bb

[linear]

MOB-1109